Process for producing an aromatic concentrate from a mixed hydrocarbon stock



w, 3,213,152 PROCESS FOR PRODUCING AN AROMATIC CONCENTRATE FROM A MIXED HYDROCARBON STOCK Filed April 15, 1965 Oct. 19, 1965 Hg RECYCLE 255 x 953; XFRACTIONATOR lNPUT GAS -FRACTIONATOR SEPARAT'ON NAPHTHALENE l DEALKYLA-' ITION UNITI uqum -1 4ooF.+ AROMATIC IT'531 CONCENTRATE Hz RECYCLE AGAS PRODUCT I DEHYDRO GAS RAFFRNATE qzumor SEPARATION V l.- W 1- z gggggg: FRACTIONATORM' U i r J w ARomm'|c.

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A TTORNE Y5 United States. Patent PROCESS FOR PRODUCING AN AROMATIC CONCENTRATE FROM A MIXED HYDRO- CARBON STOCK William H. Gammon, Ashland, Ky., assignor to Ashland Oil & Refining Company, Ashland, Ky., a corporation of Kentucky Filed Apr. 15, 1963, Ser. No. 272,954 8 Claims. (Cl. 260-668) This application is a continuation in part of my copending application Serial No. 85,242, filed January 27,

1961 and now abandoned.

This invention relates to a process for converting a mixed hydrocarbon stock into an essentially pure aromatic concentrate containing a large proportion of hicyclic aromatic compounds and not more than about paraffins.

Recently the refining industry has been provided with highly effective new processing techniques for the production of aromatics, based upon the concept of partially or completely dealkylatin-g alkylaromatics to produce benzene, toluene, xylenes, naphthalene, and heavier compounds. One example of such a process for the complete or partial dealkylation of alkylnapfhthalenes to convert them to naphthalene or lower alkylnaphthalenes is described and claimed in Gammon et al. U.S. Patent No. 3,075,022, issued January 22, 1963, entitled Process for the Production of Naphthalene. By related processing techniques, alkylbenzenes are dealkylated to produce benzene, toluene and xylenes.

For economic reasons, it is desirable that the alkylaromatic feed stocks to be converted by these new processes contain as large a percentage of alkylaromatics as practical and contain only relatively small proportions of parafiins. In actual practice it is preferred that the dealkylation feed stock be at least 95% aromatic. To the extent that parafiins are present in the dealkylation feed, they reduce the efficiency of the dealkylation process by consuming hydrogen needed for the dealkylation reaction, as they are hydrocracked to lighter parafiinic compounds. Moreover, if they are not cracked to lighter materials, paraflins will appear in the dealkylated stock as impurities which are not easily separable therefrom. The availability and and desirability of these dealkylation techniques has, therefore, presented a need for an efficient, economically practical process for producing alkylaromatic concentrates, especially alkylnaphthalene concentrates, of low parafiinic content for use as dealkylation feed stocks. The process of this application is intended to fulfill that need.

In principle, it is readily possible to produce aromatics by dehydrogenating naphthenes. Many naphth and kerosene hydrocarbon stocks contain substantial concentrations of naphthenes, and from an economic standpoint are logical dehydrogenation feed stocks. However, the naphthenes in these stocks are invariably mixed with paraifins which complicate the problem of producing relatively pure dealkylatable aromatic cencentrates from such stocks by methods other than relatively expensive solvent extraction techniques. Because many of the paraffinic compounds in these dehydrogenatable stocks have boiling points which are close either to those of the naphthenes with which they are mixed or to those of the alkylaromatics produced by dehydrogenation of the alkylnaphthenes, it is difiicult to separate either the naphthenes or the resultant aromatics from these paraffins. Undesirably large concentrations of paraffins will, if not removed, appear in the dehydrogenated stock, a result which is undesirable for reasons previously explained when the dehydrogenated stock is to be dealkylated.

3,2 13,152 Patented Oct. 1 9, 1965 Conventional catalytic reforming operations remove the bulk of these paraffins by hydrocracking them to lighter gaseous products which are readily separated from naphthenes or aromatics. However, the heavier parafiins which are thus converted to gaseous or light hydrocarbons by hydrocracking are themselves valuable materials, and their destruction is economically undesirable. For that reason, conventional catalytic reforming operations are not in general economically satisfactory techniques for producing aromatic concentrates for dealkylation. In the past, in order to obtain an aromatic concentrate of low paraffin content from an impure or mixed naphtha stock, the reforming reaction has had to be conducted at severe process conditions so that substantially all of the heavier paraflfins will be hydrocracked to gaseous hydrocarbons and removed. Operating at such severe conditions causes large quantities of coke to be formed which are deposited on the dehydrogenation catalyst and reduce its active life, thereby further disrupting process economics.

The situation is even more unfavorable in regard to bicyclic aromatics, that is, naphthene and its alkyl derivatives. As a group, these compounds boil at higher temperatures than the more common single ring aromatics, Parafiins boiling in the range of bicyclic aromatics, being relatively long chain substances, are even more susceptible to hydrocracking than parafiins boiling in the benzene and lower alkylbenzene range. As a result, even heavier production of coke accompanies the dehydrogenation of bicyclic naphthenes which are admixed with paraffins in a naphtha stock. Heretofore only very short catalyst lives have been obtainable in such processes, and consequently the need for an economical process of producing an aromatic concentrate containing bicyclic aromatics from available bicyclic naphthene-containing stocks has been very pressing.

I have discovered a process for producing aromatic concentrates of large bicyclic aromatic content and of low paraffin content, typically less thean 5%, from naphthenic stocks containing parafiins without serious hydrocracking, whereby those parafiins are recovered from the aromatics produced and are available for further use.

Briefly put, the process I have discovered comprises, fractionating from a mixed hydrocarbon feed containing naphthenes a out which has an end boiling point between about 400 F. and 450 F. and a point not higher than about 425 F., so that a substantial portion, preferably all, of the bicyclic aromatic compounds obtainable by the dehydrogenation of their naphthenic precursors will boil higher than all but a small portion, preferably 5% or less, of the parafiins in the stock. This cut is then catalytically dehydrogenated at low residence time, high space rate operating conditions, whereby at least a portion of the naphthenes present in the stock are dehydrogenated to their corresponding aromatics. The dehydrogenated stock is fractionated to produce an aromatic concentrate having an initial boiling point in the range of about 400-420 F., and a lower boiling paraffin-containing fraction. The aromatic concentrate so obtained typically consists of 95% or more mixed mono and bicyclic aromatics, with 5% or less mixed paraffins and unconverted naphthenes, depending on the nature and the end point of the particular cut selected for dehydrogenation.

By dehydrogenating at low hydrocracking conditions, the dehydrogenation of monoand bicyclic naphthenes is accomplished with little scission even of long chain parafiins, with the result that excellent catalyst life is demonstrated. Moreover, because cracking of paraffins is low and the paraflins are subsequently separated from the aromatic concentrate, they constitute a valuable byproduct which is available for further use.

In contrast to many past processes, the separation of ice the aromatic concentrate by this process does not necessitate a solvent extraction step, and a further economic advantage is thereby effected. In a modification of the basic process, solvent extraction may be utilized to separate those aromatics which are contained in the lower boiling, less aromatic fraction from the paraffins and un converted naphthenes therein, but because of the smaller quantity of liquids involved that step still involves less expense than a process wherein all of the aromatics are solvent extracted from the dehydrogenated stock.

In the drawing:

FIGURE 1 is a schematic diagram of the process I have discovered; and

FIGURE 2 is a schematic diagram of a modified embodiment of the process.

A typical naphtha stock is primarily a mixture of various paraffins and naphthenes. Some quantities of olefins and aromatics are also present, but usually not in concentrations above about 10%. These compounds boil over a wide range of temperatures which overlaps the lower end of the boiling range of the bicyclic aromatics.

In the preferred practice of this process, a naphtha stock (or an equivalent naphthene-containing hydrocarbon mixture) is fractionated to an end boiling point which is fixed by reference to the composition of the stock so that at least a substantial portion of the bicyclic aromatics obtainable by dehydrogenation of the naphthenes present in the stock will boil higher than substantially all of the parafiins present in the stock. Typically, the concentrations in a naphtha stock of paraffins boiling in the range of the bicyclic aromatics diminish with increasing boiling temperature or molecular weight. In practice, it is not usually possible to obtain a heavy naphtha which contains no parafiins boiling in the range of the bicyclic aromatics, but from an inspection and PONA analysis of a naphtha or kerosene stock it is possible to fractionate a cut containing 5% or less parafiins boiling in the range of the bicyclic aromatics, i.e. above about 420 F. The end point of a heavy naphtha prepared according to these criteria will generally be between 400 F. and 450 F., and also the 95% point of the stock will generally be less than about 420 F. ,The end point should be as high as is consistent with acceptable paraflinic content of the aromatic concentrate product. Thus, where parafiinic content doesnot increase substantially with increase in the end boiling point of the cut, fractionation to a higher end point would not cause a large increase in paraffinic content of the final concentrate, yet would effect an increase in the bicyclic aromatic content of the concentrate. The initial boiling point of the cut is not critical, and Will depend upon the use to be made of the lower boiling paraffin-containing fraction to be produced following dehydrogenation. However, since the lowest boiling bicyclic naphthene boils at 369 F., some of the bicyclic naphthenes will be eliminated from the dehydrogenation feed stock if the IBP is higher than 369 F., and will therefore not be converted to aromatics.

EXAMPLE I A typical heavy naphtha selected according to these considerations might comprise, for example, a 345-425" F. cut. The inspection of a typical such feed stock is as follows:

345-425 F. Heavy naphtha API 44.9 IBP, F 345 5 359 362 365 30 367 40 368 50 370 60 373 70 375 379 386 394 EP 425 Saturates 87 Olefins 2 Aromatics 11 From this data, it can be seen that the stock has essentially no parafiins boiling in the range of the bicyclic aromatics (boiling above 424 F.) and that the proportion of parafiins boiling above 394 F. does'not exceed 5%, even assuming all of the highest boiling 5% of the fraction to be parafiinic. Thus, virtually all of the bicyclic aromatics obtainable upon dehydrogenation of this stock will boil higher than the parafiins in the stock. After the reforming step a virtually pure aromatic stock can be separated by fractionating a out having an initial boiling point of about 420 F., and only a small proportion of parafiins will be included if the initial boiling point of the aromatic concentrate is lowered to 410 or 400 F.

This stock is catalytically reformed or dehydrogenated at high space rate-low residence time conditions. The catalyst may comprise any conventional dehydrogenation catalyst. As an example of the use of a catalyst of the platinum on alumina type, the reforming catalyst sold by Engelhard Industries, Inc., Newark, New Jersey, under their trade designation RD150, can be used to dehydrogenate the stock with low cracking of parafiins by contacting the stock with a bed of the catalyst in the presence of hydrogen, at a temperature of 820-975 F., a pressure of -1000 p.s.i.g., a weight hourly space velocity of about 520, and a catalytic residence time of 4-15 seconds.

By dehydrogenating a heavy naphtha fractionated according to these considerations at conditions of low residence time and high space rate, a substantial portion of the naphthenes present in the stock will be converted to their cor-responding aromatics Without the accompanying destruction of the bulk of the paraffins present.

Following is a particular illustration of the dehydrogenation of the 345-425 F. heavy naphtha of this example:

Weight hourly space velocity ll.4 Temperature, F 924 Pressure, p.s.i.g 306 Gas/oil molal ratio 4.0

Residence time, seconds 4.8

When nephthenes are dehydrogenated,corresponding aromatics are produced which boil higher than the naphthenes from which they were formed. Thus, in the example given above, although the dehydrogenation feed stock contains no naphthenes, parafiins, or aromatics boiling higher than 425 F., all of the bicyclic aromatics pro- N aphthene B.P., Aromatic B.P.,

n-propyleyclohexane 314 n-propyl benzene 319 n-butyl cyelohexane 358 n-butyl benzene 362 trans-decalin 369 Naphthalene 424 eis-decaltn 384 do 424 Q-methyl trans 401 {2-methyl naphthalene 466 l-methyl naphthalene.-. 473

Z-methyl trans-decalin 406 {Z-methyl naphthalene 466 l-methyl naphthalene 473 n-hexyl cyelohexane 435 n-hexyl benzene 439 Z-ethyl cis-decalin 451 2-ethyl naphthalene 496 From the table it can be seen that, in general, the bicyclic naphthenes jump some forty or more degrees in boiling temperature upon their dehydrogenation to aromatics, and thereby tend to concentrate themselves in a high boiling portion of the reformate. On the other hand, monocyclic naphthenes boiling in the range of the bicyclic naphthenes, or of comparable molecular weights, increase only slightly in boiling point upon dehydrogenation, and do not significantly tend to concentrate themselves in a high boiling portion of the reformate, although some monocyclic naphthenes do boil at temperatures overlapping those at which the bicyclic aromatics boil and therefore upon conversion produce monocyclic aromatics which boil in the range of the bicyclic aromatics.

Following the dehydrogenation step, the liquid product is separated from the gaseous products of dehydrogenation, as is indicated diagrammatically on the accompanying drawing. This may be effected in any conventional manner and need not be described in detail herein. Following separation, this gas may be recycled to the dehydrogenation unit to supply the hydrogen necessary therefor.

The high boiling aromatic concentrate is then separated from the unconverted naphthenes and paraflins (and other low boiling compounds which may be present) by simple fractionation. The dehydrogenated stock is fractionated to yield a high boiling fraction which comprises high boiling alkylbenzenes in admixture with a large proportion of bicyclic aromatics, and a lower boiling fraction which comprises parafiins, unconverted naphthenes, low boiling monocyclic aromatics and other low boiling compounds. The initial boiling point of the aromatic concentrate is critical in respect to obtaining an aromatic concentrate of low parafiin content, as parafiin content increases as the I BP is dropped below about 400 F. The IBP of this fraction should be from about 400 to 420 F.

As an example of the dehydrogenated fractions obtained by this fractionation step, the lower boiling fraction of the dehydrogenated 345-425 F. heavy naphtha of this example shows the following inspection:

API gravity 42.9 'ASTM distillation:

IBP, "F 138 242 90 376 EP 401 Saturates 41.6 Olefins 1.6

Aromatics 56.8 F-l Clear 82.5 Yield, wt. percent of feed 67.0

The 400 F. and higher boiling fraction of the same dehydrogenated naphtha has an API gravity of 17.5, a

monoand bicyclic aromatic content of 98.6 wt. percent, 1.4% olefins, and substantially no paraffinic content. The yield of this fraction, in terms of weight percent on feed, is 18.9. The yield of naphthalene precursors (bicyclic aromatics) expressed as naphthalene, in terms of weight percent on 400 F. and higher cut, is 43.4.

EXAMPLE II A 358-438" F. heavy naphtha was selected as the dehydrogenation feed stock. The inspection of this stock was as follows:

385438 F. Heavy naphtha API 43.0 IBP, F 385 5 390 EP 438 Saturates 84.9 Olefins 1.6 Aromatics 13.5

This stock was dehydrogenated at conditions as follows:

Weight hourly space velocity 5.8 Temperature, F. 937 Pressure, p.s.i.g 500 Gas/ oil molal ratio 8.0 Residence time, seconds 9.0

Hydrogen partial pressure, p.s.i 459 (Platinum on alumina catalyst, RD-) Inspection of product:

API 35.6 IBP, F 148 5 214 95 485 EP 507 Saturates 35.5 Olefins 1.5

Aromatics 63.0 Liquid yield, wt. percent of feed 85.1

This dehydrogenated product was fractionated into a low boiling paraffin-containing cut and an IBP-400 F. higher boiling aromatic concentrate.

Inspection of the lower boiling fraction:

API 46.2 IBP, F 117 10 220 30 301 50 348 70 367 90 380 EP 394 Saturates 54.5 Olefins 1.9

Aromatics 43.9

The 400 F. and higher boiling fraction of the reformate had an API gravity of 20.8, a saturate content of 9.6 wt. percent, an olefin content of 1.4%, and an aromatic content of 89.0%. The weight percent of naphthalene precursors expressed as naphthalene, in terms of Weight percent on feed, was 13.0%, or 38.8% in terms of weight percent on cut. In contrast, a 420 F. and higher boiling fraction of the reformate showed negligible saturate content, 1.0% olefins, and 99.0% aromatics. The yield of naphthalene precursors expressed as naphthalene, in terms of weight percent on feed, was 13.0%, or 60.0% in terms of weight percent on cut. This illustrates the criticality of the initial point of the fraction in respect to minimizing paraffin content of the aromatic concentrate.

In the past, it has been the practice to produce aromatic concentrates by reforming naphthenes at severe low space rate, high residence time conditions to hydrocrack parafiins therein as the naphthenes are being dehydrogenated, so that they will not be present in the product. As a typical instance of conventional past practice, the same 385-438 F. heavy naphtha referred to in Example II, upon dehydrogenation at conventional conditions, e.g., a WHSV of 2.9, a temperature of 943 F., a pressure of 511 p.s.i.g., a molal gas recycle ratio of 7.0, a residence time of 21.8 seconds, and a hydrogen partial pressure of 460 p.s.i., yielded a product as follows:

API 31.0 IBP,F 140 240 30 32s 50 371 70 41s and contained 27.4 weight percent saturates, 3.1% olefins and 69.5% aromatics. Liquid yield was 75.5%, in sharp contrast to the 85.1% yield of reformate shown by Example II. Catalyst life was extremely short.

A 400 F. and lower boiling fraction of this conventional reformate had a saturate content of 38.0%, an olefin content of 2.7% and an aromatic content of 59.3%. The yield of this fraction, in terms of Weight percent on feed, was 48.6%. A 400 F. and higher boiling fraction of this reformate was 98.4% aromatics, 0.6% olefins, 1.0% paraffins, and contained 55.6% naphthalene precursors expressed as naphthalene. The yield of this fraction in terms of weight percent on feed, was 14.9%.

By dehydrogenating a naphtha cut which has been fractionated in accordance with the criteria herein specified at process conditions of high space rate and low residence time, the dehydrogenation reactions take place in preference to the hydrocracking of paraflins, with the result that those paraffins are not materially destroyed and can subsequently be recovered from the dehydrogenated stock simply by fractionation. The total liquid product may be 10% or more larger. This figure of itself indicates less hydrocracking and, consequently, considerably longer catalyst life.

Because these paraffins are not cracked during the dehydrogenation step, they will be present in the dehydrogenated stock. It is for this reason that paraflins boiling in the same range as the bicyclic aromatics must be of low concentration in the dehydrogenation feed stock, so that they will also be of low concentration in the bicyclic aromatic concentrate. Because the bicyclic naphthenes boil about 50 to 80 F. lower than their corresponding aromatics, fractionating the dehydrogenation feed stock to eliminate high boiling parafiins therefrom does not remove an important proportion of the naphthenic bicyclic aromatic precursors.

The lower boiling mixture produced from a naphthenecontaining stock by the dehydrogenation process above described will normally contain some single ring aromatics. These aromatics constitute a valuable source of dealkylatable compounds, and may be removed from that lower boiling fraction by solvent extraction and then be completely or partially dealkylated to produce benzene, toluene or xylene, for example, as described in Paulsen U.S. Patent No. 2,951,886.

EXAMPLE III Following is a description of a modified embodiment of the process whereby a dealkylation feed stock containing both monoand bicyclic aromatic compounds is prepared from a full boiling naphtha, as illustrated diagrammatically in FIGURE 2 of the drawings.

A naphtha or other stock containing both single and double ring naphthenes is fractionated to an end boiling point such that a substantial portion of the bicyclic aromatics obtainable upon dehydrogenation of bicyclic naphthenes contained therein will boil higher than substantially all of the parafiins contained therein. This stock is dehydrogenated to convert naphthenes therein to aromatics. After removal of light gases in the dehydrogenated effluent, the liquid product is fractionated to yield at least a higher boiling aromatic concentrate containing a substantial proportion of bicyclics, and a lower boiling fraction which contains monocyclic aromatics, unconverted naphthenes and paraffins. The high boiling aromatic concentrate is charged to a dealkylation unit for conversion to naphthalene.

Aromatics in the lower boiling fraction are separated by a solvent or other extraction process, and are then dealkylated to yield benzene, toluene, or xylenes in accordance with known procedures. The raflinate from the aromatic extraction unit comprises a high concentration of parafiins together with some naphthenes, and may be charged to a dehydrocyclization unit wherein these saturates, or at least some of them, are converted to aromatics. As shown in FIGURE 2, the dehydrocyclized product is recycled to the aromatic extraction unit wherein the aromatics are separated for dealkylation.

By this modification of the process substantially all of the aromatics in a full boiling naphtha are separated therefrom as an aromatic concentrate of low paratfin content from which benzene, naphthalene and derivatives thereof can be produced. The separation of this aromatic concentrate is in part effected by simple fractionation rather than by more expensive solvent extraction techniques, which is a distinct economic advantage.

While I have described the preferred embodiment of my process herein, it will be understood that it is susceptible of variations and modifications falling within the scope of the claims which follow:

Having described my invention, I claim:

1. A process for producing an aromatic concentrate containing a large proportion of bicyclic aromatics and less than 5% of parafiins, which comprises fractionating a mixed hydrocarbon stock containing paraffins and bicyclic naphthenes to produce a fraction having an end boiling point between about 400 and 450 F. and a point no higher than about 425 F. so that a substantial portion of the bicyclic aromatic compounds obtainable upon dehydrogenation of the bicyclic naphthenes in said fraction boils higher than all but a small portion of the parafiins present in said stock, dehydrogenating said fraction over a dehydrogenation catalyst to convert at least a portion of the bicyclic naphthenes therein to the corresponding aromatics, and fractionating the dehydrogenated fraction to produce a substantially aromatic concentrate having an initial boiling point above about 400 F. and a paraflin content less than 5% and a lower boiling parafiin-containing fraction.

2. A process according to claim 1, in which the mixed hydrocarbon stock comprises a petroleum naphtha.

3. A process according to claim 1, in which the mixed hydrocarbon stock is fractionated to produce a fraction having an end boiling point between about 435 and 450 F.

4. A process according to claim 1, in which the mixed hydrocarbon stock is fractionated to produce a fraction having a 95% point between about 420 and 425 F.

5. A process according to claim 1, in which the mixed hydrocarbon stock is fractionated to produce a fraction so that a substantial portion of the bicyclic aromatic compounds obtainable upon dehydrogenation of the bicyclic naphthenes in said fraction boils higher than all but less than 5% of the paraifins present in said stock.

6. A process according to claim 1, in which said fraction is dehydrogenated over the dehydrogenation catalyst at conditions of low residence time and high space rate with only minor cracking of paraflins.

7. A process according to claim 1, in which the dehydrogenated fraction is fractionated to produce a substan- References Cited by the Examiner UNITED STATES PATENTS 2,475,977 7/49 Meier 260-668 2,920,115 1/60 Friedman 208-141 X 2,958,643 11/60 Friedman 260-668 X 3,075,022 1/ 63 Cammon et a1. 260-672 DELBERT E. GANTZ, Primary Examiner.

ALPHONSO D. SULLIVAN, Examiner. 

1. A PROCESS FOR PRODUCING AN AROMATIC CONCENTRATE CONTAINING A LARGE PROPORTION OF BICYCLIC AROMATICS AND LESS THAN 5% OF PARAFFINS, WHICH COMPRISES FRACTIONATING A MIXED HYDROCARBON STOCK CONTAINING PARAFFINS AND BICYCLIC NAPHTHENES TO PRODUCE A FRACTION HAAVING AN END BOILING POINT BETWEEN ABOUT 400 AND 450*F. AND A 95% POINT NO HIGHER THAN ABOUT 425*F. SO THAT A SUBSTANTIAL PORTION OF THE BICYCLIC AROMATIC COMPOUNDS OBTAINABLE UPON A DEHYDROGENATION OF THE BICYCLIC NAPHTHENES IN SAID FRACTION BOILS HIGHER THAN ALL BUT A SMALL PORTION OF THE PARAFFINS PRESENT IN SAID STOCK, DEHYDROGENATING SAID FRACTION OVER A DEHYDROGENATION CATALYST TO CONVERT AT LEAST A PORTION OF THE BICYCLIC NAPHTHENES THEREIN TO THE CORRESPONDING AROMATICS, AND FRACTIONATING THE DEHYDROGENATED FRACTION TO PRODUCE A SUBSTANTIALLY AROMATIC CONCENTRATE HAVING AN INITIAL BOILING POINT ABOVE ABOUT 400*F. AND A PARAFFIN CONTENT LESS THAN 5% AND A LOWER BOILING PARAFFIN-CONTAINING FRACTION. 